The present invention relates to a class D amplifier, and, more particularly, to an audio speaker amplifier circuit having pseudo noise (PN) modulation.
Class D amplifiers, also known as a switching amplifiers, are amplifiers that switch at a high frequency. Class D amplifiers use active power circuit elements, such as switches which are alternately driven to saturation and cut-off at a high switching speed, generating a rectangular waveform at its output. While the operation of traditional amplifiers is limited to increasing the voltage and current of input signals without significantly altering their waveforms (unless saturation occurs), class D amplifiers provide, prior to the amplification, the encoding of the information or audio signal using a particular duty-cycle modulation system, wherein the rectangular waveform can be modulated with a low-voltage signal within the audio bandwidth. Using Duty-Cycle Modulation (DCM) including Pulse Duty Cycle Modulation (PDM), or Pulse Width Modulation (PWM), the modulation results in a duty-cycle or a pulse width modulated waveform at its output. Conventionally, field effect transistor (FET) circuitry produces a PWM waveform, wherein the square wave or pulse frequency is set to meet the Nyquist criterion of at least twice the highest frequency to be amplified.
Class D amplifiers modulate the duty cycle or width of square wave pulses as a function of the input audio signal. When the volt-second area is identical for both the positive and negative pulses, the pulse cycle average is zero volts. This corresponds to a 50% duty cycle. By varying the duty cycle from the 50%, zero volt output state, the average output can be made positive or negative. The required analog signal for driving the loudspeaker is then obtained by appropriate filtering downstream of the final stage of the amplifier to remove the high-frequency carrier waveform and reconstruct the high-voltage, low-frequency waveform from the modulation input command.
The PWM modulation step converts the audio signal to be amplified into a sequence of pulses of the square waveform type, having a pulse duration that is proportional to an instant amplitude of the input signal. This type of modulation affords very high efficiency levels, in principle of up to 100%. The resultant signal, having a much different waveform from the original one, is complete with all information of the input audio signal. Since class D amplifiers use a fixed frequency triangle wave generator to implement the pulse width modulator, the resulting harmonics produced tend to extend beyond the 30M Hz in frequency range such that filtering is required to meet regulatory requirements of the Federal Communications Commission (FCC). The resultant filtered high-voltage waveform lies within the audio bandwidth and when applied to a speaker will produce sound.
More particularly, FIG. 1 shows a simplified block diagram of a conventional class D amplifier. An audio input 104 includes an audio signal to be amplified. A triangle oscillator 102 generates a triangle wave. Both the audio input 104 and the triangle oscillator 102 serve as inputs to pulse wave modulator 106. The digital or analog DCM or PWM modulator 106 that is responsive to a digital or analog input signal 104 and triangle oscillator 102, to produce a duty cycle or width modulated square wave. A power amplifier 108 is responsive to the duty cycle or width modulated square wave, to produce an amplified DCM/PWM square wave. The power amplifier 108 may employ both positive (+) and negative (xe2x88x92) power supplies. A low pass output filter 110 filters out Electromagnetic Interference (EMI) and the high frequency carrier waveform from the amplified DCM/PWM square wave to drive a load such as a loudspeaker (not shown) at output 112. EMI must be filtered to promote privacy since any nearby radio receiver may be capable of demodulating the signal. The conventional class D amplifier relies on post filtering to attenuate the EMI to an undetectable level and not remove the voice signal content. Thus, the resultant filter 110 is expensive due to many inductors and capacitors.
In summary, a power H-bridge-configured switching circuit 108 incorporated within the conventional class D amplifier 100, which is operative to source and sink current with respect to the output audio circuit (speaker), contains power FETs which are driven (gated on and off) by the output of the class D (pulse width modulation) amplifier, so as to effect pulse duration modulation of a pair of complementary power supply voltages for driving the speaker at the required amplification level. Harmonic energy in the switch mode signal manifests as EMI. The output of the PWM-driven power bridge switching circuit 108 is a high energy square wave-type signal, which is filtered in a downstream audio filter 110, configured as an inductivexe2x80x94capacitive network, to remove the switching transients and preserve the desired audio for application to one or more speakers (which constitute the load for the filter). The purpose of filter 110 is to reduce harmonics of the switch mode signal. If the amplifier drives a resistive load, it will produce a high ripple current that will consume power. Thus, within a conventional class D amplifier, not only must EMI be eliminated, but ripple current from a resistive load must be eliminated where applicable.
Thus, disadvantages of the conventional class D amplifier arise due to the use of the associated audio filter 110 downstream from the amplifier necessary to eliminate EMI and high frequency components within the amplified DCM/PWM square wave coupled with the fact that the performance of its audio output filter 110 is highly dependent upon the output load. In addition, these output filters 110 are conventionally implemented using inductive-capacitive networks that have a large number of poles which promote distortion.
One approach to improve the design of the class D amplifier is to introduce negative feedback into the amplifier circuit; yet, this solution, as described in U.S. Pat. No. 3,294,981 (which is incorporated by reference herein), is not capable of suppressing distortions brought about by the integrator implemented using negative feedback.
Thus, there is a need to provide a class D amplifier which eliminates the need for an associated audio filter and can provide high stability using very simple circuitry; thereby, reducing cost, circuit board area, and design time.
To address the above-discussed deficiencies of the class D amplifier, the present invention discloses an audio speaker amplifier circuit having pseudo noise modulation. A first embodiment of the amplifier in accordance with the present invention includes a frequency-hopping code generator to generate a signal for spreading processing which connects to a modulated triangle oscillator to oscillate the spreading processing signal. A modulator having an audio input port such that the modulator dynamically modulates the oscillated spreading processing signal as a function of the input signal is also included. It couples to a power amplifying switching circuit, being operative to generate a power amplified audio drive signal for application to an audio output load port in accordance with the output of the modulator. Dithering the frequency with frequency hopping code, such as a Pseudo Noise (PN) code, spreads the spectrum of the pulse width modulated (PWM) output.
Advantages of this design include but are not limited to a class D amplifier that eliminates the need for demodulating filtering. The amplifier in accordance with the present invention also provides high stability using very simple circuitry; thereby reducing cost, circuit board area, and design time.
Moreover, PN spreading or any other spreading technique reduces the peak energy over time of any one switching frequency or harmonic of that switching frequency. Further, PN spreading eliminates the possibility that any EMI produced will be demodulated by any nearby radio receiver. In an example, a cell phone with normal class-D amplifier may produce an EMI signal that is detectable and discernable by an unwelcome listener. PN spreading virtually eliminates discernability, by making all EMI act as random noise containing no voice signal. PN both lowers the peak sustained energy of any EMI harmonic, which deters detection, and it scrambles the voice content. This allows for smaller, inexpensive filters or in some applications elimination of the filter.